CATION-EXCHANGE PROPERTIES OF ANDOSOLS STUDIED WITH AMMONIUM ACETATE (PH7) AND UNBUFFERED BARIUM-CHLORIDE

A comparative study of the cation exchange properties of Andosols from the Canary Islands has been done by two methods: ammonium acetate 1N pH 7 and <<compulsive exchange>> by BaCl2. SoilS formed a chronosequence including highly weathered to vitric Andosols. Exchange properties depended on composition of allophanic components. For divalent cations, the replacing power of Ba2+ was higher than that of NH+4. This result was explained in terms of the specificity of Ca2+ for hidroxylated surfaces as, well as by the influence of adsorbed sulphate. Potassium was more efficiently extracted by NH+4, especially in soils containing more siliceous allophanes. CEC by the <<compulsive>> method and effective CEC values were well correlated. Soils with high values of CEC-NH4 frequently yielded low values of CEC-Ba and viceversa. This result was explained in terms of reactivity of the soils and field levels of pH and ionic strenght.